This week we’re returning to planning our system. Let’s rough out the system described in the previous paper then we’ll be calculating the system performance requirements. With WOSP we can produce an IRS system schematic.
As a quick recap here is the system we’ve been asked to tender for:
‘A three floor apartment block that has twelve flats on each floor and each flat requires a Quad Outlet plate in the Living Room for Sky Q, Sky HD/Plus, DTT (UHF) and DAB/FM.’
Note: Within the IRS industry this is known as the take-off (and enables us to plan our hardware requirements).
The pdfs that accompany this tutorial can be found at the bottom of the page for download (Diagrams 1 to 7 as a pdf).
In Diagram 1 we’ve shown how our floors are arranged and added 6x WSCR506 and 3x WM512. This arrangement will provide our feeds to the number outlets required (12x Sky Q dSCR feeds and 12x Legacy feeds per floor).
When planning your system always work from the bottom (furthest user outlet) back to the antenna array (Satellite Dish and DTT antenna).
In Diagram 2 we’ve added a Quad outlet plate to represent the furthest outlet in the system from our antenna array. We need to take into account the loss of the outlet plate too. Typically this is 2dB. We’ve added a label to indicate the distance in metres and in this example that’s 20m. In most instances you’d need to estimate this distance unless it can be calculated from a building schematic.
While we’re on the topic of cable losses we are always going to use the worst case scenario (expected losses at 2150MHz). This ensures that good levels under these conditions means that the system signal levels as a whole will be above the requirements. So let’s take a moment to look at this:
For the most part we will be dealing with WF100 type coaxial cable and the typical loss at 2150MHz is 0.3dB over a distance of 10m (losses will be less at lower frequencies). In our example let’s say our outlet is 20m away from the Multiswitch so we’d expect a 6dB loss over this distance.
Tip: You’ll notice that the second satellite port is supplied from the dSCR and the first is supplied from the Multiswitch. This is industry common practice in the UK where Quad outlets are specified.
In Diagram 3 we’re are going to connect these devices together to create our system. So for Floor 1 to the Ground we can split our signal using a WS52-05 -4dB splitter. Remember this value as it will come into play later.
Above that on Floor 2 we’re going to use a Tap, WT51-10 the reason for this is to pass the signal down (the trunk) with the minimum of loss -4dB and although we will have a loss of 10dB on the Tap output we can use a Launch Amplifier at the top of our system to overcome this. Here we are going using a WATS-530.
In Diagram 4 we have our full set of IRS components for this job, so let’s take a look at our signal level requirements thought the system starting at the furthest point from the antenna array.
Let’s open Diagram 5 here we can see that the signal values have been added for you to follow along. Where Levels in red are where measurements can be made, levels in blue are our losses and levels in green are where we have added gain (via amplification).
Working back from the outlet plate we can now make our calculations, we’re looking for 75dB on both of the Satellite outlets here. The loss of outlet plate is about 2dB our cable loss back to the WM512 Multiswitch is 6dB (20 x 0.3dB). At this point let’s look at the Multiswitch we need 83dB available here on the REC output. The WM512 has a variable gain of +10dB so an input level of 73dB would be ideal as we only need to set a gain of +6dB. We also have to take in account the through losses of the 2 dSCRs (-4dB each) and then the cable loss going up to our WS52-05 splitter on Level 2 which would need an output of 86dB available and an input of 90dB (the WS52-05 splitter has a through loss of -4dB).
For the dSCR part our calculations are easier, we still have the same outlet and cable losses back to the dSCR (which is -8dB combined) but, the Whyte dSCR has an ACG output of 85dB so we can still achieve a balanced output power. We should find that the levels for Sky Q (dSCR) and Legacy are more or less the same at the Quad outlet plate.
In Diagram 6 as we go up to level 2 we have 86dB of signal available from our WS52-05 splitter. As we calculate back from the Quad outlet plate we can see that a little more gain at the Multiswitch is needed +7dB in this case.
In Diagram 7 (our last diagram) as we go up to Level 3 we encounter our WT51-10 Tap, this has a through loss of -4dB and a Tap loss of -10dB, but we can still achieve our target level at the outlet and in this case the Multiswitch gain is set at +8dB. Calculating back up the chain we can see that our amplifier needs to be set at 95dB as a launch level and of the 30dB available on the WATS-530 we’re only going to be using 21dB.
Off air (from the LNB on the satellite dish) we can expect around 83dB of level so over 30m of cable we will have a loss of -9dB.
As a rule of thumb we can expect the Terrestrial levels (TERR) to be 10dB below that of the Satellite (SAT). So there we have it a system plan step by step. In your systems you will encounter different cable lengths and losses and on occasion different or existing hardware, each having their own effects on signal levels. In our next technical post we’ll be featuring a video that talks through the above process.
Click here to download the example PDF.